1. Field of the Invention
The present invention relates to generating a tracking error signal in an optical disc system, and more particularly, to an apparatus and a push-pull method for producing the tracking error signal in an optical disc system that includes a multi-divisional photo detector, wherein a signal output from the multi-divisional photo detector is filtered in order to prevent deterioration of the tracking error signal.
2. Description of the Related Art
A push-pull method utilizing a multi-divisional photo detector is extensively used as a tracking method for controlling an optical spot to be condensed on a recording surface of an optical disc and required to follow a track of the optical disc in an optical disc system.
A method of producing a tracking error signal with a conventional push-pull method using a four-divisional photo detector will now be described with reference to FIGS. 1 through 3.
FIG. 1 is a diagram of a pickup of a general optical disc system, the pickup including an objective lens 13, a polarized prism 15, and a light detecting means 17.
A laser beam output from a light source (not shown) passes through the objective lens 13 and is concentrated on a recording section of the optical disc 11. Also, the laser beam is reflected from the polarized prism 15 to be incident on the light detecting means 17.
FIG. 2 is a diagram of a four-divisional photo detector useable as the light detecting means 17 of FIG. 1. The light-receiving surface of the photo detector is divided into four sections A through D in a radial direction. The photo detector receives the laser beam that is reflected from the optical disc 11 and incident upon the light detecting means 17, and detects and outputs the intensity of the received laser beam per section.
FIG. 3 is a block diagram of a conventional apparatus for producing a tracking error signal by a push-pull method using the four-divisional photo detector shown in FIG. 2. The apparatus shown in FIG. 3 comprises first through third operation units 31, 33, and 37, and an amplifier 35. Referring to FIGS. 2 and 3, the first operation unit 31 performs an operation of (PB–PC) where PB indicates an intensity of a beam received by the light-receiving section B and PC indicates an intensity of a beam received by the light-receiving section C. The second operation unit 33 performs an operation of (PA–PD) where PA indicates an intensity of a beam received by the light-receiving section A and PD indicates an intensity of a beam received by the light-receiving section D. The amplifier 35 amplifies a signal output from the first operation unit 31 k times. The third operation unit 37 performs an operation of (R1–kR2) in order to output an output signal R3, where R1 is a signal output from the second operation unit 33 and R2 is a signal output from the amplifier 35. As a result, R3=(PA–PD)–k(PB–PC). The signal R1 obtained as (PA–PD) may contain a track cross component and a shift component of the objective lens and the signal R2 obtained as (PB–PC) may contain a small track cross component and a large shift component of the objective lens. Thus, under such conditions, if the signal R2 is multiplied by k, which is a regular gain, and subtracted from the signal R1, the track cross component remains approximately as before, but the shift component of the objective lens is removed, thereby finally obtaining the tracking error signal.
However, the apparatus of FIG. 3 is disadvantageous in that the overall output signal depends on the division ratio of the photo detector or a track pitch of an optical disc. For instance, if the widths of the light-receiving sections B and C are larger than those of the light-receiving sections A and B, the track cross component is output, as well as the shift component of an objective lens, when a laser beam crosses a track. In this case, the phase of a track cross component in the signal R1 is the same as that of a track cross component in the signal R2, and therefore, a track cross component in the output signal R3 of a tracking error signal decreases, thus resulting in deterioration of the characteristics of the tracking error signal.
Also, the signal R2 is used only to detect the shift component of an objective lens and has a lower frequency than the signal R1. If the signal R2 has a higher frequency component than the shift component of the objective lens, the higher frequency component will work as noise when calculating a tracking error signal.
In the event that an optical disc includes a defect, the defect affects the light-receiving sections B and C more than the light-receiving sections A and B. Thus, the signal R2 has noise in a light-receiving section having a defect, thereby deteriorating the tracking error signal.